I ran into some trouble making fire strikers work properly late this summer. I have been making them for a long time and all of a sudden they quit working. Using brand new 3/16" X 1/4" W1 from MSC. The trouble SEEMED to start when I got into a new 3 foot bar. After a lot of question-asking and some research and analysis, I think I have solved my problem. What do you guys know about decarburization of tool steels? Why? How much? Examples? Etc.?
I'm not telling what I think I learned until I hear from some of you.
Because the carbon goes away. In general, it's the result of heating carbon steel in air -- the hotter you get the steel, the worse the condition is. Heating it above transition temperature (Curie point) without protection from oxygen in the air will start decarb immediatelyl. The carbon combines with the oxygen. The higher the temp and the longer the part soaks at high temperature, the worse it gets.
Plain high-carbon steels (W1; music wire; AISI/SAE 1070 and above) are the worst offenders. High-alloy steels, in general, present less of a problem.
An eigth-inch layer of bark (decarb layer) is quite possible if you really screw it up.
When they used to make a lot of punches and other tools from W1, it was a major heat-treat issue. The popular solutions are heating in a carbon boat (for small-scale heat treating); heating in a dissociated ammonia or other carbon-rich atmosphere; heating in a ceramic, clay, or steel boat with charcoal or carburizing compound; and heating in a vacuum. Look up "muffle furnace," too.
It works for O-1. I smear some Ivory soap on the steel and add charcoal to the pouch for good measure. The cutting tool comes out light grey after quenching and cuts relatively hard steel like truck springs with only minimal grinding or honing.
Tin can steel works pretty well if you don't have the stainless foil. I leave one end open and cram in charcoal so the tool will shake out easily into the quench pot.
It provides a little carbon to help prevent scale and it probably helps prevent decarb, as well.
Just be aware that charcoal briquettes put out VOLUMINOUS amounts of carbon monoxide when heated. You might want to put a CO detector nearby -- not that one briquette is going to flood the room with CO, but you'll want to be aware of any concentration around the furnace.
In commercial heat-treating, CO is a commonly used source of atmospheric carbon. Where they want to simply avoid oxygen, big continuous-process heat-treat muffle furnaces often dissociate ammonia (NH3) and pump the combined N and H gases into the furnace to purge oxygen. Whether they use a CO atmosphere or a N/H atmosphere, the overflow is burned off at the furnace outlet to prevent explosions from CO or H2.
So you can avoid changing the carbon content of the steel either by purging oxygen, or by "doping" the atmosphere with some carbon to combine with any oxygen that's present in the atmosphere. Obviously, getting a neutral result that neither adds nor subtracts carbon from the steel is a tricky proposition with the carbon-rich atmosphere.
An animal fat based soap bakes into the hard protective crud layer that's so difficult to scrub off a barbecue, though quenching from red heat knocks it right off. As Boy Scouts we learned to rub Ivory on the bottom of cooking pans so the soot from the campfire would wash off easily.
I've read that a paste of flour and salt works too.
Include a layer or two of paper inside the SS foil, and crimp it hard enough to keep air from getting in easily. If there are trapped air spaces in the foil wrap, add more paper. The paper burns using the oxygen and leaves a carbon source in contact with the steel.
Makes sense. The coal should contribute more carbon, if the workpiece is kept in contact with the coal.
And, of course, if you have a small electric furnace, *and* a TIG setup, flow argon (or some other inert gas) into the oven to push out the air with its oxygen.
Except high alloy steels have the worst reputation for reasons you gave in the first part tho. The high alloy steels need longer soak times at higher temperatures and so cook off more of their carbon.
W1 etc many times can be heated quickly and quenched.
But yeah, even heated and quenched quickly there's still a reduced carbon layer of some sort when using my propane burners. In my case it's expected and I just grind it off but also I'd grind it thinner anyway since a thin knife blade cuts most stuff better. ;)
With my gun and knife springs made from 1095 or O1 the thin reduced carbon layer doesn't seem to effect anything. ?? :)
Hmmm...Ok. That could get unnecessarily complicated for what's being discussed here, but, in a broad generality, the diffusion rate of carbon in plain-carbon steel is higher than for many, if not most alloys, and thus it reaches the surface and oxidizes more quickly at high temperatures. It's also true that alloy steels containing chromium, molybdenum, and maybe some other elements require less carbon to achieve a given hardness, so, in terms of hardenability, it's a more critical issue with plain-carbon steel.
Beyond that, it's probably not important to get into the details.
True. You sacrifice some strength if you also temper it quickly, but it's done all the time in blacksmith work and in other heat treating done in a forge or with a torch, as a practical matter. And the initial heating/quenching rate itself doesn't have much effect as long as the piece has reached the transition temperature all the way through.
It affects something, but not something that may be noticed or that may matter in your applications.
A skin of decarb reduces the yield point and hardness of that surface layer, right where it matters most. However, it doesn't affect the Young's modulus, so it has no effect on spring rate -- within the elastic limit.
So a spring with a very thin decarbed surface will perform identically, as a spring, to one that's hard all the way through. It just can't be bent quite as much without taking on some degree of permanent set. In a normal application, your spring probably is operating well within the yield-strength envelope of that steel, so you'll never notice it.
Contact is not really needed. There is gas between the coal chunks, and where you are in the fire determines if there's (much) oxygen in that gas.
Nitrogen is way cheaper. AFAIK, most electric kiln/furnace elements (Kanthal or the like) depend on an oxide coating, so you may burn them out faster that way - not a problem with the stainless foil envelope and a bit of something to burn inside the envelope to use up oxygen/supply carbon.
You can't see it directly in a forge (all that coal gets in the way) but it's similar to an oxy/acetylene or propane/air flame in that there's fuel-rich and oxygen rich areas. You want to heat the steel in a fuel-rich area (not really the coal, as such, but the gases given off from the coal.) When doing small things the expensive way (OA) you use a "soft" or carburizing (fuel-rich) flame with a long feather. With a coal forge, if you get the steel too deep in the fire, or don't build the fire up with enough coal, or use to much air blast, you burn out the carbon by having the steel hot (for a prolonged time) in an oxygen-rich environment. It's obviously going to see a little of that when it's out of the fire and hot, but not for such a long time.